The present invention relates to a highly stable crystal oscillator of lead wire led-out type and constant temperature type (hereunder referred to as highly stable oscillator), and particularly relates to a highly stable oscillator which is effective in heat utilization
The frequency stability of this kind of highly stable oscillator is high, since the operating temperature of a crystal resonator is maintained constant by a thermostat. Therefore, a highly stable oscillator of this kind is used industrially in optical communication base stations for example. Recently, miniaturization has spread even to these highly stable oscillators, and corresponding miniaturization has been required.
FIG. 2A and FIG. 2B are diagrams describing one conventional example of a lead wire led-out type crystal oscillator of this kind. FIG. 2A is a vertical sectional view of the highly stable oscillator, and FIG. 2B is a perspective view of the lead wire led-out type crystal resonator used therefor.
As shown in FIG. 2A, the highly stable oscillator of this kind comprises a first circuit board 1a and a second circuit board 1b. The first circuit board 1a is supported by metallic pins 3a serving as external terminals which are insulated from and pass through a metallic base 2. A thermostat 4 and temperature control elements 5 are arranged so as to construct a temperature control mechanism that maintains the temperature of the thermostat 4 constant. The thermostat 4 comprises a metallic cylinder having one open end. A heating coil 6 is wound around the periphery thereof, and a thermistor 5a is fitted as a temperature detection element. Moreover, the thermostat 4 is secured on a power transistor 5b used for electricity control, which is disposed on the first circuit board 1a. 
The second circuit board 1b is supported by metallic pins 3b placed on the first circuit board 1a, and blocks off an upper opening face of the thermostat 4. Moreover a crystal resonator 7 and oscillating elements 8 are disposed on both opposite principal planes of the second circuit board 1b, thereby constructing an oscillating circuit. This oscillating circuit is a voltage control type, having a voltage-variable capacitive element 8a for example.
As shown in FIG. 2B, the crystal resonator 7 of this kind of crystal oscillator comprises for example an AT cut or SC cut crystal piece 7b sealed off inside a metallic container 7a (TO5 type) with five lead wires 9 leading out from its bottom. This crystal piece 7b is retained inside the metallic container 7a while maintaining its plate face horizontal, and is employed as a highly stable oscillator, particularly for communication equipment.
The lead wires 9 of the crystal resonator 7 pass through the second circuit board 1b and are secured thereon by soldering, and the metallic container 7a of the crystal resonator 7 disposed on the one principal plane is accommodated in the thermostat 4. Furthermore, the highly temperature-dependent oscillating elements 8, the characteristics of which fluctuate according to the temperature of voltage-variable capacitive elements 8a and the like, are disposed on the other principal plane of the second circuit board 1b, and are accommodated in the thermostat 4. Then they are covered with a metallic cover 10.
According to such a conventional highly stable oscillator, the operating temperature of the crystal resonator 7 is kept constant by the thermostat 4, so that frequency fluctuations in the oscillation frequency due to temperature variations can be prevented. In other words, fluctuations in the oscillating frequency based on the frequency temperature characteristics of the crystal resonator 7 can be prevented. Moreover, since the second circuit board 1b mounted with the oscillating elements 8 is disposed on the thermostat 4, frequency fluctuations due to the temperature characteristics of the circuit elements themselves are also prevented. Since highly temperature-dependent, highly heat sensitive elements such as in particular the voltage-variable capacitive elements 8a are accommodated inside the thermostat 4, the highly stable oscillator can further increase the frequency stability, for example can maintain a frequency deviation of 0.05 ppm or less. Therefore, the highly stable oscillator is employed particularly for industrial purposes.
Moreover, with the conventional highly stable oscillator, the temperature control mechanism including the thermostat 4 is disposed on the first circuit board 1a, and an oscillating circuit including the crystal resonator 7 is disposed on the second circuit board 1b. Therefore, the temperature control mechanism and the oscillating circuit can be manufactured separately, and hence their design and manufacture can be facilitated. Furthermore, the oscillating elements 8 are mounted on the second circuit board 1b, and are electrically connected to the first circuit board 1a by the metallic pins 3b. Here, the first circuit board 1a is not directly led out externally, and hence heat dissipation to the outside can be prevented (see Japanese Unexamined Patent Publication KOKAI No, Hei 01-195706).
However, since the temperature control mechanism and the oscillating circuit are separately manufactured for the conventional highly stable oscillator of the above construction, the first circuit board 1a and the second circuit board 1b are necessary. Furthermore, since the thermostat 4 that accommodates the crystal resonator 7 is used, an increase in the size of the oscillator cannot be avoided. In particular, since the first circuit board 1a and the second circuit board 1b are arranged so that they are vertically opposed to each other, there has been a problem of an increase the height dimension of the oscillator itself.
Moreover, the oscillator also has had a problem in that the oscillator itself becomes expensive since the thermostat 4 with the heating coil 6 wound therearound is used separately from the crystal resonator 7. There is an oscillator that uses the metallic container 7a of the crystal resonator 7 also for the thermostat 4. However, in either case, there has been a problem in that the manufacturing operation becomes troublesome, and the oscillator itself becomes more expensive because of a need for winding the heating coil 6 around the thermostat 4.
An object of the present invention is to provide a highly stable oscillator, the structure of which is simpler, and in particular, which is of reduced height dimensions.
Moreover, the present invention relates to a constant temperature type crystal oscillator that uses a surface mounted crystal resonator (SMD: abbreviation of Surface Mounted Device), and particularly relates to a constant temperature type crystal oscillator having a simple structure.
FIG. 6 is a diagram for explaining one example of a conventional surface mounted crystal oscillator of this kind, wherein FIG. 6A is a vertical sectional view of a constant temperature type crystal oscillator, and FIG. 6B is a schematic diagram showing a crystal oscillator inserted into a thermostat.
A crystal oscillator of this kind comprises; a crystal resonator 22, a thermostat 23, oscillating elements 24, and temperature control elements 25 disposed on a first circuit board 21a and a second circuit board 21b. The first circuit board 21a is supported by metallic pins 27a (hermetic terminals) serving as external terminals which are insulated from, and pass through, a metallic base 26. The second circuit board 21b is supported by metallic pins 27b implanted on the first circuit board 21a. Both the first circuit board 21 and the second circuit board 21b are composed of glass epoxy material.
The crystal resonator 22 comprises for example, an AT cut or SC cut crystal piece sealed off inside a metallic case 29, shown in FIG. 6B, with a pair of lead wires 28 leading out. The thermostat 23 is formed with a metallic cylinder 30 having a heating wire 31 wound therearound, and accommodates the crystal resonator 22 as also shown in FIG. 6A. Moreover, the principal plane of the metallic cylinder 30 is placed so as to face one principal plane of the second circuit board 21b, and both of these are thermally joined by a thermo-conductive resin 32. Furthermore, a pair of lead wires 28 of the crystal resonator 22 are bent and connected to the second circuit board 21b. 
The oscillating elements 24 constitute an oscillation circuit together with the crystal resonator 22, and are disposed on the other principal plane of the second circuit board 21b. The temperature control elements 25 include at least a thermistor 25a as a temperature sensitive element, and together with power transistors, construct a temperature control circuit that controls the temperature of the thermostat 23. The members apart from the thermistor 25a are disposed on the peripheral edge of the first circuit board 21a. In this temperature control circuit the temperature of the thermostat 23 is detected for example by joining the thermistor 25a to the thermostat 23. Then, based on this detected temperature, the power to be supplied to the heating coil 31 is controlled to maintain the temperature inside the thermostat 23 constant. A metallic cover 33 covers these members.
According to such a crystal oscillator, the operating temperature of the crystal resonator 22 can be controlled to be constant by the thermostat 23, so that frequency fluctuations of the oscillation frequency due to temperature variation can be prevented. In other words, fluctuations in the oscillating frequency based on the frequency temperature characteristics of the crystal resonator 22 can be prevented. Moreover, since the second circuit board 21b mounted with the oscillating elements 24 is disposed on the thermostat 23, frequency fluctuations due to the temperature characteristics of the circuit elements themselves can be prevented.
However, as shown in FIG. 6B, since the crystal oscillator of the above construction uses the crystal resonator 22 in which a crystal piece is accommodated in the metallic case 29 that has lead wires 28 leading out externally, there is an increase in the size of the crystal oscillator itself. Moreover, since the thermostat 23 with the heating coil 31 wound therearound is used, the crystal oscillator itself becomes more expensive and its structure becomes more complex. There is also an oscillator in which the metallic container 29 of the crystal resonator 22 has the heating coil 31 directly wound therearound. However, even in this case, an operation for winding the heating coil 31 on the thermostat 23 is required, and hence in either case there is a problem of increased complexity of the structure.
Moreover, these crystal oscillators are employed for use in a base station, as having a frequency stability of 0.05 ppm or less as described earlier. However, since for example, GMS purpose requires comparatively moderate frequency stability of 0.1 to 0.2 ppm or less, there have been instances of over specification. In light of this, application of a temperature compensated crystal oscillator for surface mounting may be considered. However, in this case frequency stability becomes approximately 1 ppm, and hence there is a problem in that it can not satisfy predetermined standards.
An object of the present invention is to provide a constant temperature type crystal oscillator in which miniaturization is advanced, and the structure is simplified.